10 january 2018 - nasa...magnetic signature • paleomagnetic studies of apollo samples* and...
TRANSCRIPT
10 January 2018
B Jolliff, C. Shearer, N. Petro,
B. Cohen, Y. Liu, R. Watkins,
D. Moriarty, S. Lawrence,
and C. Neal
Science Priority
• Why sample return from SPA Basin?− Important for lunar science
• impact history of the Moon
• early magmatic evolution
− Important for Solar System science
• What happened in the first 500 Myr of Solar System history?
SouthernFarside
Aitken crater
South Pole
LRO LOLA DTM on WAC context mosaic
NASA/GSFC/ASU/MIT
Ellipse center 191°E, ellipse long axis 11°counterclockwise, long axis ~2400 km
SPA Basin
Sample Return from SPA is listed in the Decadal Survey as high-priority.
Science Priority
• A unique location on the Moon and in Solar System
• largest and oldest clearly recognizable lunar impact basin
−SPA event completely resurfaced huge part of the Moon and reset ages over an enormous area.
−As such, SPA anchors the lunar impact-basin chronology.
Critical Science objective is to determine basin formation age and “SPA chronology.”
SouthernFarside
Aitken crater
South Pole
LRO LOLA DTM on WAC context mosaic
NASA/GSFC/ASU/MIT
Ellipse center 191°E, ellipse long axis 11°counterclockwise, long axis ~2400 km
SPA Basin
SPA-SR Compelling Science Questions
• What was the heavy bombardment history of the Moon?− Cataclysm? Duration?
• What are the implications for early Earth and the terrestrial planets?− Critical time for early life on Earth
(and elsewhere?)
• What are the implications for early Solar System Dynamics?− Models: Nice, Grand Tack, Pebble Accretion,
Extended decline of accretion
SPA-SR Compelling Science
• Better understanding of the impact-basin-formation process−How deep did SPA penetrate, how were the
excavated materials distributed, how did the Moon’s crust and mantle respond?
• Elucidate Crust / Mantle / Core structure−What are the processes that produced large-scale
planetary heterogeneity?
−When was the core dynamo active and at what strength?
• Thermal Evolution of the Moon−What is the distribution of heat-producing elements in the
lunar interior and implications for thermal evolution?
• Basalts as Probes of the Farside Mantle−What is the heterogeneity of the farside vs. near-side mantle?
Chemical and Mineralogical Signatures
Mineralogical zones defined by Moriarty & Pieters, 2016, LPSC 47
J. Head
Mineralogy, M3 data
Magnetic Signature
• PaleomagneticstudiesofApollosamples*andspacecraftmagnetometryofthelunarcrust**indicatethatanintensecoredynamowasactivebetween4.2-3.6Ga.− onlyonesampleanalyzedw/modern
techniquesthathasanage>3.9Ga
• Keyquestions:− Whenwasthedynamoinitiated?− Whatwasthesourceofenergythatpowered
thedynamo?Corecrystallization,precession,impacts?
• Whatmaterialsareresponsibleforthemagneticanomalies?
• DatetheSPAimpactmeltrocksanddeterminetheirremanentmagnetization!
Wieczorek et al. (2012) Science 335
LP total magnetic field strengthPurucker and Nicholas (2010) JGR 115
LRO LOLA topography
* Weiss&Tikoo,2014 **RichmondandHood,2008;PuruckerandNicholas,2000
xxx
Possible Relationship to Magmatic Activity
• Potential causal relationship of SPA to
global magmatic activity on Moon.
GRAIL gravity gradient map
Rift valleys possibly formed during an episode of crustal magmatism
Andrews-Hanna et al., 2014Schultz & Crawford
Possible Relationship to Magmatic Activity
• Recent chronologic evidence for a spike in magmatic activity > 4.3 Ga…
• What was the role of SPA? Grange et al., 2013
Figure 1: distribution of U-Pb ages of lunar zircon grains from Apollo 12, 14, 15, and 17 landing sites.
4320 Myr
Is this peak a result of magmatism associated with the SPA impact?
Answers: in Samples from SPA Basin
• Samples of SPA basin hold keys to each of these questions:−Ages−Compositions
• major elements
• trace elements
• isotopes
−Mineralogy
−Magnetic SignaturesLROC NAC oblique view of
interior of Mafic Mound, near the center of SPA basin.
NASA/GSFC/ASU
Rock fragments separated from Apollo 11 regolith
Answers require analyses in terrestrial laboratories.
Selecting a Landing Region
ManyPotentialLandingSitesinSPA
• Approachtosamplingandselectinganappropriatelandingsitedependsonscienceobjectivesandcostconstraints.− e.g.,automatedsamplereturnofregolithorsievedrockfragments
− Apollostylesortiemission
Figure:
MostoftheareashowningreencorrespondstoNectarianterraorplainsmaterials.
Diversebutmostlylow-lying,smooth-appearingterrain.
Landing Site Safety Assessment
derived from NAC DTM
Terrain Ruggedness*
*Terrain Ruggedness Index: mean elevation difference between adjacent pixels in the DTM (Riley et al., 1999; Lawrence et al., 2015)
Landing Site Safety Assessment
LROC NAC DTM Data
NAC DTM
Landing Site Safety Assessment: Boulders
LROC NAC DTM Data
NAC DTM
500 m
Bhabha “b” Landing Ellipse:12.86 km2
37 boulders within ellipseLargest boulder: 3.5 mDiviner Rock Abundance: 0.002 (0.2%)Calculated RA*: 6.4E-5 (0.00006%)
Diviner Rock Abundance comparison:
NAC image
N
b
Landing Site Safety Assessment: Boulders
LROC NAC DTM Data
NAC DTM
500 m
NAC image
N
b
25 m
Boulders > 1 mN: 37 bouldersMean: 2.1 mMax: 3.5 m
LROC NAC Geometric Stereo Coverage
• NACGeometricStereoobservationsneededforNACDTMgeneration− RequiresslewingLROSpacecraft
− Requiresspecificilluminationconditions
− Operationallyexpensive
• Goodandgrowingcoverageinkeyareas
Conclusions
• SPA Sample Return Science- Determine age of samples that date
formation of the SPA Basin.• Test models of LHB timing and causes;
establish >4 Ga lunar impact chronology.
- Numerous objectives for understanding early evolution of the Moon
• Including possible relationship between SPA formation and major igneous activity.
• Testing concepts for early core dynamo and internal structure.
- Many possible landing sites• Center of basin
• South polar regions
• Transient crater rim
Sample return - from SPA - will address fundamental questions of early Solar System history, processes of giant impacts, and internal evolution of the Moon.
LRO provides the essential data for science evaluation and landing site terrain analysis.
• Landing Site Planning and Hazard Avoidance- Highly accurate positional and
coregistered data sets • Enable detailed geologic studies to
support landing site assessments.
- Imaging and topography at unprecedented scale (0.5-1.0 m)
• Provides essential data for automated landing and safe operations.
Backup
Possible Ages from Crater Size-Frequency Analysis
Hiesinger et al., 2012,
LPSC 43
Oppenheimer
Schrödinger
Planck
Apollo
Model age of SPA basin
Relative ages of basins within SPA
Apollo: ~3.91 GaPlanck: 4.09 GaSchrödinger: 3.92 GaOppenheimer: 4.04 Ga
Problem: Chronology poorly constrained >4 Ga
Lunar Chronology: Key for planetary surface ages
Chronology:
How well do we know these ages?
Lunar Chronology: Key for planetary surface ages
Chronology:
How well do we know these ages?
No “ground truth” data between >4 Gyr!
Samples are needed for age determinations!
hardly at all
Volcanic Resurfacing in SPA: Sparse (?)
Key Questions addressed by analysis of farside basalts:à age and compositional character of
farside mantle
à mixture of basalts vs. SPA impact melt in regolith
Red and yellow: mapped mare basalts
Green: mapped cryptomare
Orange: mapped pyroclastic deposits
Pasckert et al., accepted (2018)
Rock fragments in Sample dominated by SPA substrate
Will sample abundant impact-melt rocks and breccia, as well as volcanic materials, mare and cryptomare.
Ballistic sedimentation• Mainly digs up & redistributes
material from SPA impact melt complex
• Rock material mostly SPA substrate, excavated and redistributed
• With significant (measurable) inputs from subsequent large impacts
Haskin et al. (2003) Lunar Planet. Sci. 34, #1434
Model for production of regolith by impact ejecta, showing proportions of materials contributed by various impacts
SPA
substrate
Geophysics - GRAIL
• SPA~10-20 km crustal
thickness (likely impact melt body)
• Low Porosity~ 6%
• High Density~ 2800 kg/m3
Wieczorek et al.Science (2013)
Crustal thickness superposed on topography. Model assumes crustal porosity of 12% and a mantle density of 3220 kgm−3
Ø Need to know what are the rock types!
Sieve: concentrate rock fragments; collect unsieved regolith for context
Rock fragments carry unique, individual histories of igneous, impact, and volcanic events.
Rock fragments (2-10% by mass of regolith) – represent local and distant events– rock types are diverse because of impact mixing.
Sampling capabilities:• Scoop to 10s of cm depth
• Sieve regolith to increase number of rocks by 25-50x
• 900-950 g sieved; 50-100 g unsieved
• Unsieved regolith for comparison with orbital data
Landing Site Safety Assessment: NAC DTMs
M112653051M112646261
Relief ~ 250 mBhabha - East Plains
2009_316M112653051
i=63°
31 km
BhabhaCrater
20 km
Small is Beautiful!
Example of Analyses of a Small Rock
12033,638-1
Grid: 2 mm
BSE
X-ray
R: Al; G: Mg; B: Fe
Petrography
Mineralogy
Mineral Chemistry
Chemistry: INAA: 20 mg
Rock is a mafic impact-melt breccia, rich in incompatible trace elements. New Apollo type.
Example of Analyses of a Small Rock
Crystallization Age: 3918 ± 16 Ma;
Interpretation: Age of Imbrium
Big Instrument,
Small Rock!
Spot size of primary beam: 10 μm
Zircon U-Pb Chronology